ANNIVKHSAUr ADDItKSS OF Till] I'Ui;sll)i;NT. I 29 



the other hand, there is abundant proof of the usual contact- 

 metamorphism. Though the sills conform on the whole to the 

 bedding;, they every here and there break across it. They swell out 

 into thick irregular masses, and thin out rapidly. In short, they 

 behave as true intrusive sheets, and not as bedded lavas. 



In regard to their internal character, they show the customary 

 uniformity of texture throughout each mass. They are mapped 

 under the general name of " greenstones" by the Geological Survey, 

 and are described in the Memoir as hornblendic*. The more 

 precise modern methods of examination, however, prove them to be 

 true diabases, in which the felspar has, as a rule, consolidated 

 before the augite, giving as a result the various types of diabasic 

 structure t. 



The date of the intrusion of these basic sills can be fixed by the 

 same process of reasoning as was applied to those of the Arenig 

 volcanic group. Their connexion with the other igneous rocks of 

 Caernarvonshire is so obvious that they must be included as part of 

 the volcanic history of the Bala period. But they clearly belong to 

 a late stage, perhaps the very latest stage, of that history. They 

 could not have been injected into their present positions, unless 

 a considerable mass of rocky material had overlain them. Some of 

 them are certainly younger than the tuffs of Snowdon and Moel 

 Hebog, which belong to a late part of the volcanic period. On the 

 other hand, they had been intruded before the curvature and com- 

 pression of the region, for they partake in the foldings and cleavage 

 of the rocks among which they lie. The terrestrial movements that 

 produced this disturbance have been proved to have occurred after 

 the time when the uppermost Bala rocks were deposited, and before 

 that of the accumulation of the Upper-Silurian formations J. The 

 epoch of intrusion is thus narrowed down to some part of the Upper- 

 Bala period. With this subterranean manifestation volcanic action 

 in this part of the country finally died out. 



I have already alluded to the Berwyn Hills as an independent 

 volcanic area containing two groups of interbedded igneous rocks, of 

 which the lower has been paralleled with the Arenig group of 

 Merionethshire, while the upper is shown by its association with the 

 Bala Limestone to be on the whole synchronous with the Snowdonian 

 masses. But this area has not been worked out in greater detail 



* Op. cit. p. 15G. t Harker, * Bala Volcanic Series,' p. 83. 



X Mem. Geol. Survey, vol. iii. 2nd edit. p. 326. See also Mr. Marker's * Bala 

 Volcanic Series,* p. 76. 



